1. Field of the Invention
The present invention relates to an apparatus for controlling fuel evaporated from an internal combustion engine, and particularly, to one that is capable of securing the working capacity of the engine and of properly controlling an air-fuel ratio when the purging of evaporated fuel from a fuel tank into the engine is resumed.
2. Description of the Related Art
A canister having an activated carbon layer is used to adsorb evaporated fuel produced in a fuel tank of an internal combustion engine. The adsorbed fuel is released from the layer by air passed through the layer and is supplied into an intake duct of the engine. If the canister is used alone, the adsorbing capacity, i.e., working capacity thereof will be insufficient to adsorb evaporated fuel, and a large amount of evaporated fuel will be purged from the fuel tank into the engine when the engine is started, thereby deteriorating the driveability of the engine and the quality of the exhaust gas. To solve this problem, Japanese Unexamined Utility Model Publication 63-198462 has proposed an apparatus for controlling evaporated fuel employing a main canister and a sub-canister connected in series.
As the level of fuel in a fuel tank approaches the bottom, the fuel tank produces a lot of evaporated fuel, which is adsorbed by the main and sub-canisters of this prior art system up to the working capacities thereof. As a result, the canisters may have no space to accept more evaporated fuel. When the fuel tank is replenished with fuel and the engine is restarted, the main canister releases the adsorbed fuel into the sub-canister that is full of adsorbed fuel. Accordingly, a large amount of released fuel is supplied into the engine.
To solve this problem, Japanese Unexamined Patent Publication 6-10996 of this applicant has proposed an apparatus for controlling evaporated fuel of an internal combustion engine, which guides, when a fuel tank is replenished with fuel, evaporated fuel from the fuel tank into a main canister, and when the engine is restarted, into an intake duct of the engine through an evaporated fuel path that bypasses the main canister and a sub-canister. This prior art is capable of securing the working capacities of the canisters because no evaporated fuel is guided from the fuel tank to these canisters during the operation of the engine.
According to this prior art, an adsorption material in the sub-canister adsorbs evaporated fuel around a path between the sub-canister and the intake duct during the operation of the engine. This results in changing the quantity of fuel adsorbed by the sub-canister between a given purge operation and the next purge operation. Namely, the concentration of fuel to be purged into the engine is unknown when a given purge operation is started. If the concentration of purged fuel is unclear, it is difficult to correctly calculate a fuel injection quantity that is essential to carry out feedback control to attain a target air-fuel ratio. This deteriorates the driveability of the engine and the quality the exhaust gas.
To solve this problem, Japanese Unexamined Patent Publication 5-52134 has proposed an apparatus for controlling fuel supply to an internal combustion engine, which gradually opens a purge control valve that guides fuel released from a canister into an intake duct of the engine. If the purge control valve is fully opened at the start of the purging of fuel from the canister into the intake duct, an air-fuel ratio will be too rich and will deteriorate the driveability of the engine and the quality of an exhaust gas. Accordingly, this prior art gradually opens the purge control valve until the concentration of purged fuel is correctly detected.
This prior art system takes a long time to detect the concentration of purged fuel. During this period, the purge control valve must gradually be opened, and therefore, the canister must hold a large amount of adsorbed fuel. Namely, it takes a long time to release the adsorbed fuel from the canister and recover the working capacity thereof. In the meantime, the adsorbed fuel uniformly diffuses through an activated carbon layer of the canister, to reduce the concentration of adsorbed fuel in the layer. Then, the adsorbed fuel hardly leaves the layer, deteriorating the working capacity of the canister. As a result, a large amount of evaporated fuel is purged into the engine at the start of the purge operation, to provide a rich air-fuel ratio and deteriorate the driveability of the engine and the quality of the exhaust gas.